It is known to use sun or solar collectors in combination with heat pumps to convert radiation energy of the sun into useful heat. (See Journal "Elektrizitatsverwertung" No. 3/1975). The heat collector may consist of tubes laid on a roof surface having heat transmitting fluid circulating therein. The fluid may consist of water or salt water. When it leaves the collector, fluid is supplied to the heat pump which withdraws heat therefrom. Known collector plants for operations take up radiation energy. At the same time, heat is again discharged by convection to the environment because the temperature of the collector plants is higher than the ambient temperature.